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Thread: Odd interaction: AudioInputI2S, AudioSynthWaveform, AudioRecordQueue, AudioPlayQueue

  1. #1
    Junior Member
    Join Date
    Jan 2022

    Odd interaction: AudioInputI2S, AudioSynthWaveform, AudioRecordQueue, AudioPlayQueue

    I am having a weird issue with these four elements. Here is my audio design tool setup:

    Click image for larger version. 

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    The Teensy and audio shield are connected to and powered from my computer. A guitar is going into channel 1 of the mixer using the left line in feed on the audio shield. The synth stuff going in to channel 2 of the mixer is just a square wave set to turn on and off automatically by LFO. The effect2In and effect2Out parts go to my effects processing loop where I am applying a very simple 10000 sample long delay effect with no feedback. Here are the issues I'm noticing:

    1. So right away there is a lot of noise on the line coming from the i2s input. I could probably use a pre-amp or something, but still this noise level is way above what I should normally expect. It sounds kind of like white noise with maybe a very faint high pitched whistling. This noise remains when I disconnect entirely from the audio in pins on the shield.
    2. I hear the square wave playing regularly. I also hear the echo from the delay. However, I *also* hear a third very faint repeat of the sound as if there was a very small amount of feedback to the delay (which I don't see in my code). The timing of this faint echo changes when I change the delay time from 10,000 samples to something else -- i.e. it's always on time as a feedback delay.
    3. If I remove the I2S input from the code, the faint extra echo on the synth goes away and the synth sound has just one echo like I would expect.

    I am wondering if anyone else can verify whether or not it happens on their Teensy 4.1 + Audio Shield as well because this seems so strange to me that I am wondering if maybe my teensy and/or audio shield are damaged. Maybe I am just doing something wrong with the code though. Does anyone see anything? I'm pretty stumped here.

    Here is the source code. I spent all day stripping it down to the bare minimum from a much larger codebase:

    #include <Audio.h>
    #include <Wire.h>
    #include <SPI.h>
    #include <SD.h>
    #include <SerialFlash.h>
    // GUItool: begin automatically generated code
    AudioSynthWaveform       waveform2;      //xy=81,122
    AudioSynthWaveformDc     dc1;            //xy=91,162
    AudioSynthWaveform       waveform1;      //xy=214,76
    AudioMixer4              mixer1;         //xy=234,142
    AudioInputI2S            audioIn;        //xy=357,70
    AudioEffectMultiply      multiply1;      //xy=367,114
    AudioMixer4              synthMixer;     //xy=516,89
    AudioRecordQueue         effect2In;      //xy=661,89
    AudioPlayQueue           effect2Out;     //xy=787,88
    AudioOutputI2S           audioOut;       //xy=927,87
    AudioConnection          patchCord1(waveform2, 0, mixer1, 0);
    AudioConnection          patchCord2(dc1, 0, mixer1, 1);
    AudioConnection          patchCord3(waveform1, 0, multiply1, 0);
    AudioConnection          patchCord4(mixer1, 0, multiply1, 1);
    AudioConnection          patchCord5(audioIn, 0, synthMixer, 0);
    AudioConnection          patchCord6(multiply1, 0, synthMixer, 1);
    AudioConnection          patchCord7(synthMixer, effect2In);
    AudioConnection          patchCord8(effect2Out, 0, audioOut, 1);
    AudioConnection          patchCord9(effect2Out, 0, audioOut, 0);
    AudioControlSGTL5000     sgtl5000_1;     //xy=752,135
    // GUItool: end automatically generated code
    int16_t delayBuffer[44100] = {};
    int writeIndex = 0;
    int readIndex = 34100;
    void processAudioEngine() {
    void setup() {
      synthMixer.gain(0, 1.0);
      synthMixer.gain(1, 1.0);
      // Calls to AudioRecordQueue.being must always be last in this function to avoid latency.
    void loop() {
      if (effect2In.available() >= 1) {
        while (effect2In.available() > 1) {
        int16_t *audioBuffer = effect2Out.getBuffer();
        memcpy(audioBuffer, effect2In.readBuffer(), 256);
        for (int i = 0; i < 128; i++) {
          int16_t nextSample = audioBuffer[i];
          delayBuffer[writeIndex] = nextSample;
          nextSample = nextSample + delayBuffer[readIndex];
          audioBuffer[i] = nextSample;
          if (readIndex >= 44100) {
            readIndex -= 44100;
          if (writeIndex >= 44100) {
            writeIndex -= 44100;
    Thanks in advance for any help!

    Last edited by kenjib; 02-25-2022 at 04:12 AM.

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